In this experimental setup, no statistically significant differences could be detected in any biofilm parameters during inter- or intra- flume comparisons. Furthermore, biofilm formation as well as biostabilization were very similar under the same boundary conditions. It has to be emphasized how crucial this result is for all further considerations as this constituted the scientific proof for the reliability of this mesocosm: The fact that identical abiotic boundary conditions resulted in very similar biofilms was an essential prerequisite for the investigation of different influencing factors – as seen in the sections 3.2 and 3.3 – and allowed a first elucidation of some fundamental principles of riverine microbial biostabilization.
1. Microbial biostabilization is not only of essential relevance in marine and brackish habitats but also in lotic systems. Compared with biofilms from intertidal mudflats, biofilms in the presented flume mesocosmos displayed lower biomasses. Reasons for this may be higher nutrient levels and longer maturation of the biofilms on the researched brackish or marine sediments. However, throughout the experiments in the flume mesocosmos, a high degree of biostabilization could be detected in developing biofilms– even under boundary conditions of minimal nutrient supply - as inert glass beads were used as substrate and the river water had a minimal nutrient load. The tested sets of environmental conditions allowed only a first
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but very important insight in the huge variety of different possible combinations of boundary conditions in the environment as these results demonstrated the significance of microbial stabilization of fine sediments in rivers.
2. The extracellular polymeric substances (EPS) play an essential role during the process of biostabilization. Presently, it is generally assumed that EPS are the main responsible factor for the gluing and stabilizing effect of biofilms on fine sediments. As carbohydrates generally contribute the majority to the EPS, the main focus of many studies laid on further investigations of these components e.g. by analysing sugar monomers or functional groups.
However, in the experiments of the present work, correlations between EPS protein content and biofilm adhesiveness were clearly higher than between EPS carbohydrate content and biofilm stability. This indicated the importance of extracellular proteins e.g. adhesive proteins produced by sessile diatoms despite their lower quantitative proportion in the EPS matrix compared to extracellular carbohydrates. In addition, this observation may highlight the different roles of carbohydrates and proteins in the extracellular matrix where small amounts of secreted proteins e.g. extracellular enzymes produced by bacteria can play an important structural and functional role.
3. High microbial biomass and EPS production enhances biostabilization as especially microalgae produce high amounts of EPS with stabilizing effect. High correlations between biofilm adhesiveness and algal biomass as well as bacterial cell counts could be detected which is supported by various studies describing the link between microbial biomass, the produced EPS, and resulting biostabilization. However, as the dominant species of algae and bacteria could clearly be correlated to different levels of biofilm stability, the composition of the microbial community appears to be of greater significance for biofilm stability than mere biomass. Especially a dominance of sessile bacterial and diatom species might be beneficial for the stability of the biofilm matrix while motile specimen can have a disrupting effect on the biofilm system. Furthermore, the high correlation between functional organization of the bacterial community and adhesiveness in matured biofilms emphasize the role of the symbiotic interactions of bacteria and diatoms, and the resulting shaping influence upon the bacterial community composition.
4. The seasonal changing microbial community has a significant impact upon biofilm development and corresponding biostabilization. The seasonal changing microbial biocenosis in the water column of rivers in moderate climate, e.g. the succession of diatoms,
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green algae and cyanobacteria during the course of the year, is well known. The investigations described in section 3.2 could demonstrate how this seasonality significantly influenced especially the algal community of the developing as well as matured biofilms. In this context, it was very remarkable that a change of pioneer species to later successional stages in the diatom community was already sufficient to induce a significant decrease in biofilm adhesiveness. Reasons for this development might be found in the different lifestyles of early and late successional stages and different requirements of adaptation during the course of the year e.g. the need and possibility for fast reproduction in a situation without macrophyte competition during spring versus high competition and predation risk during summer and autumn. Thus, these results should be taken into account during further experiments aiming towards a deeper investigation of impact factors on biostabilization in rivers.
5. Abiotic boundary conditions – especially light intensity- have a significant impact upon the microbial development, production and sediment stabilization as photosynthetic microbes dominate the metabolic pathways in the biofilm system. The two investigated abiotic boundary conditions – light intensity (LI) and bed shear stress (BSS) – clearly differed in their influence on biofilm development. On the one hand, algal biomass and biofilm adhesiveness displayed clear rising values with increasing levels of LI. Especially the very low biofilm stability detected under minimal LI growth conditions demonstrated the essential role of algal EPS production as a nutrient source for heterotrophic microbes during the process of biostabilization in this flume system where no additional nutrient sources existed.
On the other hand, increasing levels of BSS delayed biofilm development and biostabilization. The microbial community might be able to adapt to a rather unstable environment under high levels of BSS to a certain degree, as indicated by the constantly high dynamics in the bacterial community and high quantitative fraction of mobile diatom species that were detected in the biofilm matrix. However, as high biostabilization seems linked to earlier successional stages it remains questionable if a considerably longer experimental duration can result in comparable stabilizing effect as observed under the lowest level of BSS.
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